The blower on my Amana/Goodman unit started to shut itself off intermittently, despite fan being set to "on" at all times on the t-stat.You can hear the 'clicking' from the board as it tries to start the fan, but it is not successful.

However, if the AC is turned lower, the AC starts and the blower starts up again, and stays on after the AC has stopped (once temp is achieved).

This problem happenned VERY seldom at first, but I placed a service call to get it looked at, as it is such a new furnace.

On the first service call the fan had stopped to they "saw" it first-hand (the clicking and not re-starting). They couldn't figure out the problem exactly (tested t-stat as well). They placed an order for a new circuit board.

On the second service call, the tech replacd the board, but left all the dip switches in the factory defaults, which led to far too high a CFM setting. This did not fix the blower issue either, as it shut itself off 2 days later. I restarted it by activating the AC, and then again it shut off 2 days later. The next day it cycled through start/stop/start/stop every 30 seconds for about 5 minutes, and then eventually stayed on. The problem seems to be getting worse.Given that the service tech left the board at the factory defaults, I'm not eactly all that confident in their ability to fix this issue. I was able to set the dip switches myself to the correct settings as per the initial installation 2 years ago.

On the 3rd service call the blower hadn't shut off, and the tech still was stumped. He also said that the new circuit board didn't appear to have switches to adjust the blower speed. I'm just a homeowner, and with some research and reading the manuals I was able to fugre it out myself.... so again, not a lot of confidence in these techs. He ordered a new blower motor, which has yet to be installed.

It feels like these techs are just guessing, and in the meantime my 2 year old furnace is being filled with replacement parts, which doesn't seem "ideal".

Can anyone offer a suggestion for why the fan would randomly shut itself off, but seem ok restarting when the AC kicks in?

My first suggestion is to measure the AC voltage at the air handler control board where the thermostat wires connect between the G and C terminals. With the fan set to ON you should always have 25 volts, AC present. When the fan stops with the fan set to ON at the thermostat, verify you have 25 volts AV between G and C.

If you have 25 volts between G and C with the fan not running (but fan at the thermostat set to ON) then the issue is either the control board or blower motor. I assume all fan relays are on the control board. With the motor not running and 25 volts present between G and C, carefully feel the motor case. Is it very hot? Possibly the thermal limit is stopping power. One other easy part to replace is the motor run capacitor, if it's a PSC motor.

If you do NOT have 25 volts between G and C (but fan at the thermostat set to ON) then the issue is the thermostat or the wiring between the thermostat and air handler. Measure the AC voltage at the thermostat between G and C and see if there's 25 volts.

A different tech came this time and did a much more thorough diagnotic of the problem.

He did in fact tie it back to something faulty in the thermostat, but it wasn't exactly obvious what it was. The thermostat is 16 years old. Testing the voltage did show 25+ volts and a back EMF, but the blower still only worked when bypassing the t-stat. With a new thermostat installed, were up and running, with no blower shutdowns (so far).

He took the unecessary replacement blower motor that was delivered, but left the replacement circuit board in (and I also still have the original, which I'm sure is also fine and never needed replacing in the first place). As he said, since its under warranty, the service company didn't care about the costs as it was passed on to the manufacturer. A bit annoyoing, since manufacturer expenses are always passed back to us consumers eventually.

We will se if in fact this was all fixed, as it had been "fixed" before, only to fail 2 days later.